The present invention relates generally to semiconductor devices and more specifically to such devices which have reoxidized nitrided oxide regions.
Until recently, various semiconductor devices, such as N and P type MOSFETs which include a polysilicon gate overlying a gate oxide, were formed with gates composed of regular oxide material, i.e. silicon dioxide gates in devices constructed from a silicon substrate. There is a strong trend toward scaling down the size of such devices without compromising performance capabilities.
One practice that has been employed in the manufacture of scaled down devices is the use of re-oxidized nitrided oxide material for gate oxides. It has been shown that the use of re-oxidized nitrided gate oxides (ONO gates) provides a significant improvement in gate oxide quality with respect to charge generation due to high field and radiation, retardation of boron diffusion from boron doped polysilicon gates, and hot electron resistance in both P and N-MOSFETs. Additionally, the high positive fixed charge at the edge of the polysilicon gates, which results from the use of ONO gates, beneficially increases the punch through voltage for P-MOSFETs.
It has been determined that the foregoing benefits do not all flow directly from the ONO gate, but also from the nitrogen region which forms in the substrate and the gate oxide along their mutual interface as the result of forming the ONO gate. The nitrogen region forms in both the substrate and in any overlying nitrided oxide material along the interface of the substrate and the overlying nitrided oxide, which may include all or part of the gate oxide. In prior art devices this nitrogen region is uniform along the interface of the gate oxide and the substrate, and typically has a nitrogen concentration level of upwards of 10-20% by atomic weight.
There are some disadvantages, however, associated with the ONO gates of the prior art and the accompanying underlying nitrogen regions. For example, the uniform ONO gates of the prior art result in a uniform high fixed charge density along the interface of the gate oxide and the substrate (i.e. over the entire width of the gate oxide). The high fixed charge density under the interior or center of the ONO gate increases scattering and thus detrimentally degrades mobility between the gate oxide and the substrate.
It has now been determined that nitrogen concentration levels of greater than about 4% will cause mobility to be unacceptably reduced. It has further been determined the nitrogen concentration levels of about 4% or somewhat less under the gate oxide provide adequate hot electron resistance and retardation of boron penetration from a polysilicon gate into the substrate. It is also known that a nitrogen concentration level of about 10% or greater is required under the periphery of the gate oxide to provide a high enough positive fixed charge density to sufficiently increase the punch-through voltage for P-MOSFETs.
Therefore, an improved and preferred device with an ONO gate may be formed such that the nitrogen region has a nitrogen concentration level of no more than about 4% under the center of the gate oxide and a nitrogen concentration level of at least about 10% at and beyond the periphery of the gate oxide. Other improved, but less preferred devices may be formed simply by providing a nitrogen region with a comparatively reduced level of nitrogen under the gate oxide, or in some instances by providing a nitrogen region which only partially extends under the gate oxide from the periphery towards the center thereof.
The aforementioned improved semiconductor devices having ONO gates may be formed by nitriding the gate oxide after the formation of a polysilicon gate above the gate oxide. The presence of the polysilicon gate may provide partial or near total shielding of the gate oxide underlying the polysilicon gate during the nitriding step (and also during the re-oxidizing step). The shielding during nitriding may reduce the concentration of re-oxidized nitrided material over the width of the gate oxide and particularly at the center thereof. Likewise, the shielding reduces the concentration of nitrogen in the nitrogen region underlying the gate oxide in direct proportion to the reduced concentration of re-oxidized nitrided material overlying the nitrogen region. Furthermore, the shielding may protect the gate oxide from being contaminated by unwanted particles during the nitriding and re-oxidizing steps.
Accordingly, it is an object of the present invention to provide a novel method of nitriding a gate oxide in a semiconductor device.
It is another object of the present invention to provide a novel method of selectively forming a nitrogen region with a non-uniform concentration of nitrogen in a semiconductor device.
It is yet another object of the present invention to provide a novel method of selectively forming a nitrogen region in a semiconductor device with a non-uniform concentration of nitrogen over the width of a gate oxide and having a minimum level of nitrogen under the center thereof.
It is still another object of the present invention to provide a novel method of forming a polysilicon gate above a gate oxide prior to re-oxidizing and nitriding the gate oxide.
It is a further object of the present invention to provide a novel method of protecting a gate oxide from contamination during nitriding and re-oxidizing processes.
It is yet a further object of the present invention to provide a novel device having a nitrogen region with a non-uniform concentration of nitrogen under a gate oxide.
It is still yet a further object of the present invention to provide a novel device having a non-uniformly nitrided and re-oxidized gate oxide.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.